1. Field of the Invention
This invention relates to the preparation of modified organic polyisocyanates and is more particularly concerned with the preparation of carbodiimide-containing polyisocyanates and with the products so obtained.
2. Description of the Prior Art
The conversion of organic polyisocyanates to the corresponding carbodiimides by heating said polyisocyanates in the presence of appropriate carbodiimide-forming catalysts is well-known; see, for example, U.S. Pat. Nos. 3,056,835 and 3,157,662. These latter processes generally give rise to complete conversion of the organic polyisocyanates to the corresponding carbodiimides due to the high level of activity of the catalysts employed.
It is also known to convert an organic polyisocyanate only partially to carbodiimide thereby obtaining a polyisocyanate containing carbodiimide groups. This result has been achieved simply by heating the organic polyisocyanate at elevated temperatures for relatively long periods (see U.S. Pat. No. 3,152,162), or by reacting the organic polyisocyanate with a monomeric carbodiimide (see U.S. Pat. No. 3,267,137) or by heating the organic polyisocyanate with a relatively mild catalyst such as a trialkyl phosphate (see U.S. Pat. No. 3,384,653). However, these various methods of preparing carbodiimide-containing polyisocyanates all suffer the disadvantage that prolonged heating of the polyisocyanate is necessary. This gives rise to undesirable side effects particularly a darkening in color of the product. This problem could be overcome by using a much more reactive catalyst to produce the carbodiimide but, unfortunately, the use of such catalysts results in complete conversion of isocyanate to carbodiimide as it is generally not possible to stop the reaction at an intermediate stage.
U.S. Pat. No. 3,761,502 describes the use of tris(chloromethyl)phosphine oxide as catalyst in the production of carbodiimide-containing polyisocyanates. This particular catalyst is said to have the advantage that lower temperatures and shorter reaction times can be used in the conversion and that the formation of carbodiimide can be halted at any desired time by cooling the reaction mixture to below 30.degree. C. However, the process suffers the disadvantage that the resulting product still contains the catalyst. The presence of the latter may interfere with the outcome of subsequent reactions, such as the formation of polyurethanes. In addition the continuing presence of the catalyst in the product can result in continued, if very slow, conversion of isocyanate to carbodiimide while the product is being stored before its ultimate use. Since the formation of carbodiimide involves the elimination of carbon dioxide, there is thus a potential hazard involving build up of pressure in the closed vessels in which the product is being stored.
Very recently, but subsequent to the making of the present invention, a process has been described in which carbodiimide-containing polyisocyanates are prepared by heating the organic polyisocyanate in the presence of a polymeric catalyst which is the salt of a polymer containing basic amino groups with a 1-oxo-phospholane-phosphoric acid. At the end of the reaction the polymer catalyst is removed by filtration or like means. The use of these catalysts still requires relatively high reaction temperatures (of the order of 150.degree. C. or higher) and lengthy reaction times.
We have now found that carbodiimide-containing polyisocyanates can be prepared using certain polymeric catalysts which permit the use of relatively low reaction temperatures and very short reaction times but which can be removed readily from the reaction product thereby preventing subsequent carbodiimide formation on storage of the product. Other advantages of the process of the invention will be apparent from the following description.